1. FIELD OF THE INVENTION
The present invention relates to a method for increasing the strength, especially the flexural strength, of a fresh concrete, a cement paste, a mortar or the like (hereinafter referred to as "cement composition"), which comprises incorporating a water-soluble epoxy resin, an aliphatic amine and a cement dispersant into the cement composition and hardening the composition.
2. DESCRIPTION OF PRIOR ARTS
Hardened products of hydraulic cements are generally poor in the flexural strength and the flexural strength at the age of 7 and 28 days is about 1/8 to 1/10 of the compressive strength. Polymer-incorporated cement compositions have heretofore been investigated as products having an increased flexural strength, and known methods for production of these products are roughly categorized into the following 4 types.
(1) Method in which hardened concrete is impregnated with monomer
A molded structure is first prepared from a cement composition, and after the molded structure is completely dried at 160.degree. C., it is impregnated in vacuo with polymerizable monomer such as methyl methacrylate, styrene or the like and a catalyst. Then, the monomer is polymerized by the action of radiation, heat, light or the like. According to this method, good products having high compressive strength and flexural strength can be obtained, but this method is defective in the point that it is impossible to make the products easily at a construction site.
(2) Method in which polymer fibers are mixed in fresh concrete
In this method, the operation of mixing polymer fibers in fresh concrete is very difficult, and a large quantity of water is required for mixing and the amount of entrained air increases accordingly. Further, the adhesion of the polymer fibers to the cement is low and therefore, both the compressive strength and the flexural strength of the products decrease.
(3) Method in which fresh concrete is mixed with polymer emulsion
According to this method, the products having an improved flexural strength can be obtained, but in order to attain a substantial improvement effect, it is necessary to incorporate the polymer emulsion in an amount of at least 15% by weight of the cement. Accordingly, the amount of entrained air increases because the emulsifiers contained in the emulsion entrain air into the products, resulting in a reduction of the compressive strength. Therefore, this is not preferred from the practical of view.
(4) Method in which monomer is mixed in fresh concrete
This method is most preferred but it is not practically applicable because both the compressive strength and the flexural strength are fairly lowered.